Approximately 750,000 new strokes occur in the United States every year and cause about 250,000 deaths (Kittner et al., J. Am. Med. Assoc. 264:1267-1271, 1990). While the human suffering caused by stroke is enormous, both to the victims and their families, the economic costs are enormous as well. Long-term follow-up studies show that most stroke survivors experience permanent disability ranging from loss of vocational competence (71%), to requiring assistance with daily care (31%), to institutionalization (16%) (Gresham et al., N. Eng. J. Med. 293:954-959, 1975). Based on these data, roughly 300,000 persons permanently lose some function each year because of stroke.
The fundamental hypothesis in stroke research is that ischemia produces disability and death, not directly, but rather indirectly by initiating a cascade of cellular processes that eventually lead to neuronal death (Pulsinelli et al., Annals Neurol. 11:499-509, 1981; Choi, Trends Neurosci. 11:465-469, 1988). Until physicians can regenerate functional neurons to replace dead ones, the best hope for stroke victims is to intervene quickly with treatments that interrupt and reverse the cascade of events triggered by the primary ischemic event before they become irreversible.
The cascade of events begins about three to four minutes after ischemia: the first step is that the concentration of extracellular excitatory amino acids increases by 10- to 100-fold (Mayevsky, Brain Res. 524:1-9, 1990; Mitani and Katoaka, Neuroscience 42:661-670, 1991). These excitotoxic amino acids trigger a subsequent chain of events that includes calcium release from intracellular stores and eventually the expression of new genes. Dead neurons and irreversible loss of cognitive and behavioral function are results of this cascade which occurs ours after the initial ischemia.
A goal of anti-stroke treatment is to intervene in the cascade of neuronal death before it becomes irreversible, saving as many neurons as possible. A substantial body of work indicates that this theoretical possibility is a realistic goal. For example, several naturally occurring proteins can prevent neuronal death after excitotoxic damage in vitro or after experimental ischemia in vivo (Berlove et al., Soc. Neurosci. 17:1267, 1991; Shigeno et al., J. Neurosci. 11:2914-2919, 1991). These proteins (including nerve growth factor, brain derived neurotrophic factor, basic fibroblast growth factor, ciliary neurotrophic factor, and others) derive from two structurally related protein families, neurotrophins and cytokines, and are involved in the control of neuronal differentiation in the central and peripheral nervous system. The most likely mechanism by which these proteins protect neurons from ischemia seems to involve the expression of various genes. Presumably those gene products inhibit a cell death program which is triggered by the excitotoxins, and which could involve calcium release from intracellular stores. One of the most interesting previous findings shows that some of these neurotrophic factors can protect neurons from death when applied up to tens of minutes after the injury (Shigeno et al., 1991).
Other examples of compounds used to treat the neurodegenerative effects of cerebral ischemia include U.S. Pat. No. 5,559,095, which describes a method of treating ischemia-related neuronal damage using omega-conotoxin peptides and related peptides which bind to and block voltage-gated calcium channels, and U.S. Pat. No. 4,684,624, which describes treatment using certain opioid peptides. These peptides are not related to neurotrophins or cytokines.
While the neuroprotective effects of the neurotrophins are encouraging, their potential clinical application is limited by their large size (10 kD or greater) which prevents effective delivery through the blood-brain barrier (BBB). Neuroprotective molecules that can cross the BBB to act on neurons imperiled by cerebral ischemia will be more efficacious in the treatment of stroke. Molecules that protect neurons against the ischemic effects of stroke will also be useful for treating Alzheimer's disease, as well as the memory deficits that are characteristic of the aging process.